1. Field of the Invention
This invention relates generally to a method for purifying a crude dimethyl naphthalene dicarboxylate and more particularly concerns a method for purifying a crude dimethyl naphthalene dicarboxylate produced from a naphthalene dicarboxylic acid which is produced by the liquid-phase oxidation of a dialkylnaphthalene or partially oxidized derivative thereof in a solvent.
2. Discussion of the Prior Art
Fibers and films produced from polyethylenenaphthalate have improved strength and thermal properties relative to fibers and films produced from polyethyleneterephthalate and are especially useful in applications such as tire cords, magnetic tape backings and hot-fill containers. Naphthalene dicarboxylic acid, especially 2,6-naphthalene dicarboxylic acid, is employed as a monomer in the production of polyethylene naphthalate and is typically prepared by the catalyzed, liquid-phase oxidation of a dialkylnaphthalene, especially 2,6-dialkylnaphthalene.
The presence of impurities in the naphthalene dicarboxylic acid can obviously have a serious adverse effect on the physical or chemical properties or performance characteristics of any formulation containing the naphthalene dicarboxylic acid itself or any polymer formed from the naphthalene dicarboxylic acid. In addition, impurities in the naphthalene dicarboxylic acid can adversely affect polymerization processes to which the naphthlene dicarboxylic acid is subjected. Such impurities in the naphthalene dicarboxylic acid formed by the catalyzed, liquid-phase oxidation of a dialkylnaphthalene or partially oxidized derivative thereof are often organic impurities or by-products formed during the oxidation and inorganic impurities corresponding to metal components of the catalysts employed in the oxidation or formed therefrom.
Thus, removal of such impurities from the naphthalene dicarboxylic acid is highly desirable. However, the removal of organic and inorganic impurities from aromatic polycarboxylic acids formed by the catalyzed, liquid-phase oxidation of polyalkyl aromatics is typically very difficult, and the removal technique employed depends on the specific aromatic polycarboxylic acid from which the impurities are to be removed and the specific oxidation conditions and catalyst employed to make it.
In particular, naphthalene dicarboxylic acids are especially difficult to purify because of their low solubility in many solvents. Formation of the methyl ester is the best method known for purifying a naphthalene dicarboxylic acid. However, formation of the methyl ester alone does not afford a sufficiently pure monomer. Therefore, the methyl ester itself must be further purified, which purification can be time consuming and involve relatively complex reaction schemes.